Multiple material copies merger
Question 1
Just pick one of your materials, rename it accordingly.
Then pick all objects who should have the same material where the last one in selection is the object with your renamed material. Hit Ctrl - L and pick &quot;materials&quot;
This replaces all materials with the last one.
Question 2
Every object can have multiple material slots (1) with names and these slots can pickup the actual material (3).
In Blender it is possible to rename the slot and/or replace the content (material) of the slot.
To do so just go to the material properties panel and select the material you wish to assign (4).
Notice that if you look at the info row (2) you can see how often a material is used  and also make sure does not get deleted, if it is nowhere assigned.